linux-kernel - Re: [PATCH v4 2/4] dmabuf: Implement copy_file_range callback for dmabuf direct I/O prep

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Message-ID: <aD7y1PgUyd-xkS1u@infradead.org>
Date: Tue, 3 Jun 2025 06:04:20 -0700
From: Christoph Hellwig <hch@...radead.org>
To: wangtao <tao.wangtao@...or.com>
Cc: sumit.semwal@...aro.org, christian.koenig@....com, kraxel@...hat.com,
	vivek.kasireddy@...el.com, viro@...iv.linux.org.uk,
	brauner@...nel.org, hughd@...gle.com, akpm@...ux-foundation.org,
	amir73il@...il.com, benjamin.gaignard@...labora.com,
	Brian.Starkey@....com, jstultz@...gle.com, tjmercier@...gle.com,
	jack@...e.cz, baolin.wang@...ux.alibaba.com,
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	bintian.wang@...or.com, yipengxiang@...or.com, liulu.liu@...or.com,
	feng.han@...or.com
Subject: Re: [PATCH v4 2/4] dmabuf: Implement copy_file_range callback for
 dmabuf direct I/O prep

On Tue, Jun 03, 2025 at 05:52:43PM +0800, wangtao wrote:
> +static ssize_t dma_buf_rw_file(struct dma_buf *dmabuf, loff_t my_pos,
> +	struct file *file, loff_t pos, size_t count, bool is_write)
> +{
> +	if (!dmabuf->ops->rw_file)
> +		return -EINVAL;
> +
> +	if (my_pos >= dmabuf->size)
> +		count = 0;
> +	else
> +		count = min_t(size_t, count, dmabuf->size - my_pos);
> +	if (!count)
> +		return 0;
> +
> +	return dmabuf->ops->rw_file(dmabuf, my_pos, file, pos, count, is_write);

So despite claiming in the cover letter that dmabufs can't support
direct I/O you are just reimplementing it badly here using a side
interface.